I'm seeing differing capitalization for the title "La Voie lactée" or "La Voie Lactée" and need to know which is more correct, or absolutely correct. The English equivalent for the title would be "The Milky Way" Many scientific, astronomical, cinematic, and other websites offer variant capitalization, and perhaps varied when within a sentence as opposed to a title. I am interested in the proper capitalization as a title, as of a book or article. I previously used the word title in my original question, but may have not stressed it enough.
In general, we only capitalize the first word in a proper noun (unless one of the next words also requires capitalization). For example, "la République française", "l'Assemblée nationale" (but "le Royaume de Belgique" because "Belgique" is itself a proper noun). So the proper way would be "la Voie lactée". An adjective would only be capitalized if it is before the noun it qualifies (e.g. "le Nouveau Testament").